It appears that melee cannot attack units that are above them (while on the damaged wall) so if you place your ranged units just at the edge, you prevent them from being able to attack you. Also, (not sure how to repeat this but…) there were two archers that were able to hang out for quite some time (as you can see in the video) without anybody laying a finger (or mace or polearm) on them.
(sorry for the compression, just wanted to keep the video size small)